Answer: Something is damaging the roots. Possible culprits could be: too much water (excludes oxygen and results in root death), nematodes (parasitize roots and reduce efficiency severely), soil borne fungal wilts (invade the plants water conducting tissues and plug them up).
The solution to too much water is obvious. Nematodes appear as swollen areas along the roots. Fungal root wilt diseases are diagnosed by cutting lengthwise down the basal stem of an almost dead plant. Look for a greyish/brown streak between the outer green bark and the inner light green stem tissues. Both nematodes and fungal wilts are prevented by planting varieties that are resistant (look for the VFN after their name).
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